Solar Install on 5th wheel

HRTKD

Boondocker
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Apr 24, 2020
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Somewhere South of Denver
For LiFePO4 batteries, my preference is to not run any cables between them. Each battery should have equal access to the rest of the system.

The way you have it wired, if the battery on the left reaches a high voltage disconnect level first, then the charge to the second battery is cut off by the BMS. This could leave your second battery always at a lower state of charge than the first battery.
 

corn18

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Sep 9, 2021
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Well, I have heard folks tell me both ways. I picked this one because they were adamant on irv2 forums that this was the right way. Sure wish I could figure out the right answer.

How does the first battery cut off the second battery? It's not like the second battery goes through the first battery. Both batteries have a direct connection to the same + and - bus bars. If the first battery shuts down, the second battery is still connected to the system, just 4 inches further down a 4/0 pipe. I figure with 4/0 cable, 4 inches long, it really doesn't matter much. That's .0074 volts drop. Until someone offers me qualitative data that indicates which way is best, I'll stick with this.
 

HRTKD

Boondocker
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Apr 24, 2020
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7,467
Location
Somewhere South of Denver
The BMS inside the battery prevents the charge from coming through that battery. Therefore the second battery will not see the charge. This is seen in a series connection for sure. I'm thinking about the process for a parallel connection and I'm not sure it applies the same way for that case.

I chose to run cables to each battery in my system. There are no cables run between each battery.
 

corn18

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I have decided to run 2 sets of 10ga wire from the roof to the batteries. For when I decide to add more panels. Easier to do it all at once.
 

corn18

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I got the 4x10ga wires run from the roof through the sewer vent hole and down to my power center. Pretty easy and no new holes. For clarification, I did not run the wires down the inside of the poop shute. Forest River cut the hole big enough to fit the wires in without additional cutting. I made no addtional holes in the roof.

I'll install the 2x200W panels I have in series on the door side of the roof. That's usually the more clear side of the roof when we camp. Then I have to decide what to do with the other side of the roof. I could over panel the single 40A MPPT with another 200W panel in series. Without tilt, it probably will never get more than 400W with 3x200W panels.

Or I can add 2x200 panels in series to feed a second 40A MPPT.

Then I could add 2 more 200W panels and do a 3S/2P setup. That should fully drive both 40A MPPTs.

Does this ever end?DC wiring 290RL.jpg
 

corn18

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Everything is installed and working. Completely overcast day and getting 75W.

I assembled everything in my garage on the strut channel and then just hauled the whole assembly up my giant ladder to get it on the roof. Aluminum strut channel sure weighs a lot less than steel, so it was easy. I used 1/4 x 1.5" stainless lag bolts to fasten the strut channel. Being able to tilt the panels sure made it easy. I used 4 lag bolts / strut into solid cross members. So 8 penetrations for 2 panels. I can tell you that fastening them just to the thin luan would not work well. And taping them to the rubber membrane would not work.

I have 6 points of attachment / panel (2 front, 4 amidships for the tilting mechanism).

I think for any additional panels I will just buy the tilting mechanism vs. making it. I enjoyed fabricating the mounts, but don't want to do it again.

Some pics below. I think I'll start another thread to determine how to expand.

IMG_4959.jpg
IMG_4960.jpg
IMG_4963.PNG
 

Zwy

Photon Sorcerer
Joined
Jan 3, 2021
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Location
Timbuktu
Well, I have heard folks tell me both ways. I picked this one because they were adamant on irv2 forums that this was the right way. Sure wish I could figure out the right answer.

How does the first battery cut off the second battery? It's not like the second battery goes through the first battery. Both batteries have a direct connection to the same + and - bus bars. If the first battery shuts down, the second battery is still connected to the system, just 4 inches further down a 4/0 pipe. I figure with 4/0 cable, 4 inches long, it really doesn't matter much. That's .0074 volts drop. Until someone offers me qualitative data that indicates which way is best, I'll stick with this.
I'd prefer equal length cables to each battery from the busbar. In your current wiring, the resistance of the cable and connections can lead to uneven discharge and charge rates from each battery. Anytime you run batteries in parallel, it is preferred to have equal cable length to a common busbar. Granted, the length is short but it still will lead to some imbalance.
 

corn18

Village Idiot
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Sep 9, 2021
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428
I'd prefer equal length cables to each battery from the busbar. In your current wiring, the resistance of the cable and connections can lead to uneven discharge and charge rates from each battery. Anytime you run batteries in parallel, it is preferred to have equal cable length to a common busbar. Granted, the length is short but it still will lead to some imbalance.
I understand the math and the general practice. But really, are you concerned about .0074 voltage drop? Show me the math on how much imbalance that could possibly result from that.
 

Mike Jordan

Solar Addict
Joined
Jun 15, 2020
Messages
412
For LiFePO4 batteries, my preference is to not run any cables between them. Each battery should have equal access to the rest of the system.

The way you have it wired, if the battery on the left reaches a high voltage disconnect level first, then the charge to the second battery is cut off by the BMS. This could leave your second battery always at a lower state of charge than the first battery.
He has the system pos off of one batt, and the system neg off of the other batt. Equal and balanced.
 

corn18

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PowerPoint. I earned my PowerPoint ranger badge with gold star when I worked at megacorp.
 
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